1. Field of the Invention
The present invention relates to a method and a system for measuring a surface of an object. In particular, the present invention relates to a method and a system for measuring a surface of an object by successively measuring different portions of the surface by an interferometric technique.
Further, the present invention relates to an optical system for shaping a light beam. In particular, the present invention relates to an optical system for shaping a light beam comprising two diffraction gratings.
2. Brief Description of Related Art
A shape of a surface of an object, in particular an optical element, such as a lens or a mirror, can be measured by an optical interferometric method. Conventionally, a beam of measuring light is generated comprising wavefronts corresponding to a target shape of the surface of the object to be measured. Thus, the beam of measuring light is incident substantially orthogonal to the surface of the object to be measured. The beam of measuring light being reflected from the surface of the object to be measured is superimposed with reference light and the resulting interferogram as detected by a spatially resolving detector. From the interferogram deviations of the shape of the surface of the object from the target shape can be determined.
Forming of the measuring light in order to comprise wavefronts having a shape corresponding to a target shape is in particular difficult, when the surface of the object to be measured has an irregular shape and a large extension. The optics for forming an appropriate beam of measuring light are also commonly referred to as K-systems, null lens arrangements or compensators. For measuring large surfaces of an object a so-called “stitching methodology” has been developed as described in the article by M. Bray, Proc. SPIE, Vol. 3492, 946 (1999). From this article a method of testing a large mirror having a spherical shape by performing interferometric tests at a plurality of overlapping portions or sub-apertures of the optical surface is known. Each portion has a lower diameter than the surface to be measured. Measured surface data of each portion are then stitched together to generate surface data representing a map of the surface shape of the tested mirror.
From WO 2005/114101 A1 an interferometric method employing the stitching methodology is known to measure an aspherical optical surface having a rotational symmetry.
The conventional methods and systems however do not allow to accurately measure aspherical surfaces, in particular they do not allow to accurately measure aspherical surfaces having a large extension.